The typical manufacturing process for memory hard disk media involves plating an aluminum disk substrate with a layer of nickel phosphorus alloy. Polishing this nickel alloy's surface with a fine polishing slurry obtains a smooth and flat surface. This polished nickel-plated substrate is then suitable for application of a layer of magnetic storage media, such as used for hard disks. The increasing demands for higher storage capacity in hard drive manufacturing have necessitated a substantial increase in areal density, i.e., data storage capacity per unit surface area, on the disk media. This requires that significant improvements be made in the manufacturing of the rigid hard disks, including enhanced plating uniformity, reduced surface roughness after polishing, and enhanced texturing characteristics. The polishing process is one of the critical factors required to fulfill these new requirements.
Significant improvements in surface inspection metrology have allowed disk manufacturers to inspect for small surface defects previously undetectable. This technology advancement has led to optimization of polishing parameters to reduce defects including: polishing cycle time; polishing pressure; and resolution rate of upper and lower table of the polishing machine. Optimizing polishing parameters requires a high degree of expertise and is time consuming. Other advances have been made in the area of consumables, such as, polishing pads, abrasive slurries, and cleaning materials. Unfortunately, conventional aluminum oxide slurries (5 to 50 m.sup.2 /g, surface area and 1 to 10 .mu.m mean diameter, size distribution) create micro-scratches and micro-pits on substrates' surfaces. Because these slurries create these defects, it is difficult for disk manufacturers to obtain smooth surfaces using conventional alumina slurries, i.e., a roughness of less than 3 .ANG., which is preferred for good deposition of a magnetic layer.
Conventional polishing slurries can result in uneven plating of the magnetic layer after polishing the substrate. Since the clearance between magnetic heads and the magnetic layer is less than 0.2 .mu.m, small surface defects on the magnetic layer such as nodules may crash and damage the magnetic heads. Other defects, such as scratches and pits, cause errors in reading or writing information on hard disks. There are several possible causes for these defects, including: 1) the surface morphologies of the aluminum oxide abrasives are irregular or contain sharp edges where the grinding action of these abrasives on the substrate's surface introduces polish scratches; 2) the presence of unwanted fine aluminum oxide particles generated from the abrasive's size reduction process causes micro-pits; and 3) the agglomeration of the aluminum oxide particles in the polishing slurry and in the pores of the polishing pads causes scratches or pits in the substrate's surface. For the above reasons, it is difficult for disk manufacturers to achieve a defect-free and low roughness surface, i.e., Ra (peak-to-valley height) of less than 3 .ANG. with conventional alumina-based slurries.
The increasing demands of the computer hard disk industry for defect-free and low roughness surfaces have forced slurry manufacturers to explore alternative polishing agents, such as, solution derived colloidal metal oxide materials. The mean diameters of these colloidal particles are typically in the range of 0.01 to 1 .mu.m; and these small and soft particles potentially offer enhanced substrate surface characteristics. However, slurry manufacturers are currently encountering two problems with colloidal slurries. First, the polishing rates of these slurries are substantially lower than conventional alumina-based slurries--disk manufacturers utilizing colloidal based slurries have to increase polishing cycle time, slurry consumption and even the number of polishing machines in order to maintain the required production throughput. Second, these colloidal particles also have a strong tendency to aggregate, coagulate and gel due to their small particle sizes, high ionic strength, and the low pH range. Therefore, colloidal slurries often have a short or insufficient shelf life.
Manufacturers have attempted to use smaller and/or softer alumina-based abrasive particles and different chemical additives such as chelating agents and oxidizers to reduce or eliminate surface irregularities. Furthermore, slurry manufacturers have attempted to use various unstable oxidizers such as hydrogen peroxide, aluminum nitrate and ferric nitrate to enhance the polishing removal rate. These oxidizers cannot be premixed with the slurry; and disk manufacturers must add these oxidizers, such as ferric nitrate, at their point of use. The use of ferric nitrate is also undesirable because it stains the polishing equipment.
Other manufacturers have used various unconventional abrasives such as boehmite and fumed metal oxides to achieve smooth substrates surfaces. Kodama et al., in U.S. Pat. No. 5,575,837, disclose the use of a persulfate accelerator with a silica sol or gel. In addition, Streinz et al., in PCT Pub. No. 98/23697 disclose the use of a triple salt of 2KHSO.sub.5.KHSO.sub.4.K.sub.2 SO.sub.4 (approximately 50% monopersulfate) or hydrogen peroxide oxidizers with a ferric nitrate catalyst for use with fumed silica and alumina dispersions for polishing hard disk substrates.
Dromard et al., in U.S. Pat. No. 5,418,273, disclose the use of anionic dispersing agents, such as sodium polyacrylate or polymethacrylate. These dispersing agents stabilize an aqueous suspension of anhydrous alumina and silica for papermaking industry coatings. Some slurry manufacturers have attempted to stabilize basic colloidal silica slurries, which is stable over an extended period of time. Although the suspension is stable, the polishing rate is substantially slower than conventional acidic alumina slurries. Adding various chemical oxidizers such as hydrogen peroxide, aluminum nitrate and ferric nitrate to these slurries increases their polishing rate.
These oxidizers either reduce the shelf life of the slurries or they become unstable before they reach the end-users. Another alternative is to add the oxidizer solution to the slurries at the point of use; however, it is undesirable for disk manufacturers, because special handling and storage facilities for the hazardous materials are required at end-users' sites.
It is an object of the invention to provide a slurry for reducing surface roughness of metal substrates.
It is a further object of the invention to provide a slurry for accelerating the polishing process.
It is a further object of the invention to provide a stable colloidal polishing slurry with enhanced polishing characteristics and minimized surface defects.
It is a further object of the invention to provide a polishing slurry having improved polishing performance on Ni-P plated substrates for computer hard disks.